Open szankerhqs opened 7 years ago
What you describe is a fundamental limitation of the hybridization expansion QMC algorithm in general. Or, at least, of the sampling/measurement method most implementations use.
Please, have a look at the equation (9) of our paper: https://arxiv.org/pdf/1507.00175.pdf
This is the accumulation formula for G(\tau)
.
Non-vanishing contributions to G_{\alpha\beta}(\tau)
, where \alpha\neq\beta
, involve a matrix element M_{ij}
such that \alpha=\alpha_i
and \beta=\alpha'_j
.
If the hybridization function is diagonal, no elements with said property can exists in matrix M
. Therefore, there is no way for an off-diagonal element of G_{\alpha\beta}(\tau)
to obtain a finite contribution.
@HugoStrand Should we add this question to the FAQ?
Calculating Green's function's for interacting clusters with off-diagonal hopping we run into strange behavior of CTHYB. For a hybridization function without off-diagonal elements the off-diagonal elements of Green's functions after solving are zero although the local Hamiltonian supports off-diagonal hopping.
A simple example is the 'complex_Gtau_ED.py' test. Setting the off-diagonal elements in the hybridization to zero this behavior occurs. Changing the original Hamiltonian in the test (first two orbitals are the impurity)
H_mat = np.array([[-0.2 , -1.4 , 0.5 , 0.4 ], [-1.4 , -0.3 , 0.4 , 0.5 ], [ 0.5 , 0.4 , 0.1 , 0.0 ], [ 0.4 , 0.5 , 0.0 , 0.0 ]])
toH_mat = np.array([[-0.2 , -1.4 , 0.5 , 0.4 ], [-1.4 , -0.3 , 0.4 , 0.5 ], [ 0.5 , 0.4 , 0.1 , 0.0 ], [ 0.4 , 0.5 , 0.0 , 0.0 ]])
and solving for the impurity Green's function, the off diagonal elements S.G_iw['ud'][0,1] vanish (see code below for working example).